The invention relates to a process and an apparatus for the production of a low-shrinkage aliphatic polyamide yarn, in which polyamide is extruded through a spinneret to form filaments, then cooled and combined to form at least one yarn, the yarn being subjected to drawing in a multi-stage process and then relaxed. The invention further relates to a low-shrinkage aliphatic polyamide yarn.
High-tenacity polyamide yarns are suitable for a large number of technical purposes, such as, for example, for use as reinforcement in tyres or conveyor belts, for use in plastics-coated materials and in cables or for the production of airbags. In order to produce such yarns economically, the product properties should have a combination of high tenacity and low shrinkage on heating. Especially desirable properties are a tenacity of at least 80 cN/tex and a hot air shrinkage of less than about 5.0%. That combination of properties is difficult to achieve, especially in a combined spinning and drawing process at high speed in industrial mass production.
The starting material used for the production of airbag yarn is usually polyamide 6.6 or PA6.6, an aliphatic polyamide composed of monomeric units derived from hexamethylenediamine and adipic acid, which has a melting point of around 260° C.
The use of the significantly more readily available polyamide 6, a polyamide composed of monomeric units derived from caprolactam, as starting material for an airbag yarn had the disadvantage of a relatively low melting point, meaning that on deployment of the airbag the yarn could more easily be destroyed by the flying sparks. That was the case in the past, because high-temperature inflation systems were used. Since the deployment of airbags nowadays takes place at lower temperatures, the use of polyamide 6 is now a possibility.
EP 2 205 780 B1 describes a spin-draw process for the production of airbag yarn from PA 6.6, wherein a multifilament yarn is created from molten polyamide by means of a spinneret, which multifilament yarn is first subjected to multi-stage drawing at a temperature of 160° C. to 245° C., then relaxed and subsequently wound up under tension. That process is not suitable for PA 6, because the holding temperature during the drawing is above the melting point of PA 6. The process is not suitable for creating a yarn having suitable hot-air shrinkage from PA6.
EP 1 666 647 B1 describes a process for the production of airbag yarn from polyamide, wherein a multifilament yarn composed of polyamide 6.6 is produced by means of melt-extrusion, the yarn is subjected to a high degree of drawing at low temperature in a first step and to a low degree of drawing at high temperature in a second step, then relaxed and subsequently wound up. That process is likewise unsuitable for processing PA 6 on account of the high temperature, since it uses a drawing temperature of up to 250° C. and a relaxation temperature of up to 260° C. Furthermore, the production of airbag yarn from PA6.6 in accordance with the above-mentioned processes is unsatisfactory, because the values for the tenacity and the hot-air shrinkage are subject to too much variation and do not yield the required values.